1. Field of the Invention
The present invention relates to liquid crystal displays, and more particularly to such displays which have a bistable characteristic.
Reports of the Bunsen Society, Volume 9, 1974, pages 912-914 describes a liquid crystal arrangement in the form of a matrix with a plurality of locations or visual elements which can be identified by unique row and column positions. The display unit uses a liquid crystal with a positive anisotropic dielectric constant.
The liquid crystal material used with the matrix display demonstrates the bistability effect under certain boundary conditions. When the applied electric field sees a threshold value of E.sub.cn the material passes out of its energetically stable (focal-conical) structure into a homeotropic-nematic orientation. This orientation is maintained until the electric field is reduced below a second threshold value of E.sub.nc, with a brief formation of a planar-conical intermediate state. A value of hysteresis is exhibited between E.sub.cn and E.sub.nc values, and the ratios of the field intensities for these threshold values can be as high as 3 to 1. Under certain circumstances, it is possible to make the lower threshold value equal to zero. In this special case, in which the characteristic cholesteric helical structure must be in a specific relationship to certain parameters of liquid crystal substance, not only a homeotropic-nematic texture but also a planar-conical one are stable next to each other in a field-free state. Both phases can be transformed into the other. For example, if the susceptibility anisotropy of the liquid crystal substance changes its sign as a function of frequency, a change of frequency can transform one phase into the other.
If an intermediate electric field is maintained between the two threshold values, the state of any portion of the liquid crystal (LC) display can be maintained for long time periods, and in increase in the so-called multiplex ratio is realized. The multiplex ratio is the ratio between the possible storage time and the required write-in time.
In practice, the time in which the elements of the LC display can be maintained in a given state is restricted, primarily because of the fact that the energetically stable focal-conical phase gradually intrudes into the homeotropic-nematic phase. This begins at locations where impurities are located, and gradually invades the nematic region.
One way of increasing the time during which a state may be maintained is by the especially careful cleaning of the plate surfaces, in order to reduce and eliminate as much as possible the impurities which may be present. It is also been suggested to hold the electric field at certain parts of the display at a constant value, so that the liquid crystal remains nematic at these locations. However, no electrode structure has heretofor been suggested for producing such a field.